In many cases there is a need to extract timing information from a received radio signal. The timing information may comprise for example a train of successive and temporally equidistant timing pulses such as e.g. a pulse per second “PPS”-signal. Furthermore, the timing information may comprise a Time-of-Day “ToD” value which represents a clock time reference that is common to two or more entities which can be, for example but not necessarily, routers and/or other network elements of a data transfer network. The received radio signal can be for example a satellite radio signal transmitted by a satellite of a Global Navigation Satellite System “GNSS” or a radio signal transmitted by a terrestrial radio station. The GNSS can be, for example but not necessarily, the US Global Positioning System “GPS” or the Russian GLONASS-satellite system.
A radio signal transmitted by a satellite of a GNSS has typically a circular polarization in which the electric field of a passing electromagnetic wave changes direction in a rotary manner. If the electromagnetic wave were frozen in time, the electric field vector of the electromagnetic wave would describe substantially a helix along the direction of propagation. The handedness, i.e. the rotational direction of the electric field vector when seen along the direction of propagation, is typically reversed when the radio signal is reflected at a surface so that the direction of arrival is sufficiently near to the perpendicular of the surface. Upon such reflection, the rotational direction of the electric field vector remains unchanged but the direction of propagation is reversed. Thus, the handedness is reversed. An inherent advantage of the circular polarization is that the reception of the radio signal is not so sensitive to the orientation of the reception antenna than in conjunction with for example a radio signal having a linear polarization.
When extracting timing information from a radio signal of a GNSS, the main challenge is typically the multi-path propagation due to reflections. The multi-path propagation causes that a device for extracting timing information receives differently delayed components of a radio signal. A known solution to this problem is to use a circular polarized antenna whose reception attenuation is strongly dependent on the handedness of the circular polarization of a received radio signal. For example, in case where the GNSS transmits a radio signal having the Right Hand Circular Polarization “RHCP”, the most powerful reflected components which are reflected once have the Left Hand Circular Polarization “LHCP” because the reflections typically reverse the handedness. Thus, the most powerful reflected components can be attenuated by using a Right Hand Circular Polarized “RHCP” reception antenna.
The above-mentioned approach based on a circular polarized reception antenna works well when the circular polarized reception antenna is a rooftop antenna which has a clear view of the sky. However, in an indoor environment or in a challenging outdoor environment, reflected components of a GNSS-radio signal can be stronger than a directly propagated component of the GNSS-radio signal. A challenging outdoor environment can be for example a deep urban canyon where direct visibility of GNSS satellites is rare or even impossible. The directly propagated component may have penetrated walls and/or a roof, and thus the directly propagated component may have been attenuated significantly. A reflected component may have entered a building via e.g. a window and then reflected at a wall or the floor. Thus, the attenuation of the reflected component can be significantly less than that of the directly propagated component. Therefore, in an indoor environment, the use of a circular polarized reception antenna may lead to a situation where the reflected component which may have a sufficient power is strongly attenuated by the reception antenna and, on the other hand, the directly propagated component which is received more effectively may be too weak for providing reliable timing information. The use of an antenna without circular polarization, i.e. handedness, will attenuate both left and right handed signals by a further 3 dB and additionally will require other mechanisms to mitigate multipath effects.